A great many products used by consumers, businesses and organizations degrade rapidly unless stored in controlled environments. These products include food, pharmaceutical and chemical products. All polymeric products and many non-polymeric products are subject to aging effects to varying degrees, and often under multiple complex degradation mechanisms. Prematurely degraded products may pose health and safety risks to customers. Products properly stored may be disposed of prematurely due to conservative shelf-life dating.
A number of methods and sensors have been developed to indicate the degraded state and/or the remaining useful life such as product expiration dates and in-situ degradation sensors. Expiration dates have been used for some time and provide a low-cost and user-friendly interface. However, expiration date methods assume environmental specification conformance within predetermined ranges and provide no feedback or date correction should the actual environmental conditions be exceeded. Product manufacturers must compensate for unknown environmental exposure by conservative product expiration dating; wasting product life, which could otherwise be used if the actual environmental exposure is known. On the other hand, consumption of spoiled food or drugs that have lost their efficacy poses serious health risks and associated costs. Many materials such as solid propellants used in aerospace applications may degrade quickly in extreme environments, making shelf life dating unsuitable.
Another type of degradation sensor used for determining degradation of products is the time-temperature integrator (TTI). TTIs may take the form of analog devices such as degradation sensors utilizing chemical reactions resulting in color changes. The color changes correspond to an integrated time-temperature condition of the product or package and provide a more flexible method to indicate product expiration as compared to date stamping. However, these devices are difficult to vary the response for different degradation models and are of limited use where direct observation of the sensor is difficult. In addition, color change indicators are difficult to use with automated or degradation communications systems.
Another approach for determining remaining product life is the digital time-temperature integrators. These devices utilize a temperature sensor such as a thermocouple or thermistor and a digital clock. Time and temperature are logged at predetermined time intervals and stored in a memory. A microprocessor, either co-located with the data logger or separate from the datalogger, integrates the time-temperature data in an algorithm modeling the degradation factors of the product. These devices offer great flexibility of degradation modeling since virtually any degradation mechanism may be modeled by a microprocessor once the necessary time and temperature data has been collected. However, these devices suffer the disadvantage of requiring power for the data logging and timing functions, and microprocessor power if integrated in the device. Power requirements prevent practical use of these devices in many automated or wireless system applications such as passive RFID tags. The complexity of the devices also makes integration in individual product packaging difficult and the cost is relatively high.
An improved method is needed to monitor the condition of degradable products that overcomes the drawbacks of current methods.